2
$\begingroup$

In cosmology, we have a couple of different length measures. Among them, comoving and proper distnlances seem to be good fit to real world use cases in astronomy.

So my quesion is which length measure are we using in astronomy? For instance, when we say a certain galaxy is # light yrs distant from the Earth, what does it actually mean?

Thanks!

Improve this question
$\endgroup$
1
  • $\begingroup$ In astronomy it seems to me that we intend proper distances. When the objects are too far the only sensible value is z. The rest comes from the model and parameters used. $\endgroup$
    – Alchimista
    Commented Aug 31, 2020 at 10:38

2 Answers 2

Reset to default
1
$\begingroup$

comoving distance is not something that we can measure. We are actually measuring either luminosity distance or angular diameter distance. And they are related to the comoving and proper distances.

By making observations we can determine the luminosity distance of an object $d_L$. And the $d_L$ is related to the transvverse comoving distance $r$ via $$d_L = r(1+z)$$ For flat universe $$r = c\int_0^z dz/H(z)$$

Improve this answer
$\endgroup$
2
  • $\begingroup$ I think this answer is too thin given that it is apparent that the OP is touching upon these issues for the first time. For example, how is the comoving distance related to angular diameter distance? One has to assume that we somehow know the diameter of the object, but if we do we can understand that distance. Comoving distance is more abstract. All this needs to be addressed. $\endgroup$
    – garyp
    Commented Oct 22, 2020 at 11:39
  • $\begingroup$ @garyp We mostly use angular diameter distance for CMB and BAO measurements. We can know their angular seperation on the sky (physics allows us to calculate it) and by measuring the angle we can refer to an angular diameter distance. In general of course we dont know the diameter of the furthest galaxies. $\endgroup$
    – seVenVo1d
    Commented Oct 22, 2020 at 13:36
0
$\begingroup$

As distances in space between objects are very very large so we cannot use normal units for measuring distances like meter, miles, kilometers etc. So the most common we use is light year. Light year basically is the distance travelled by light in one year. As speed of light is 300000km/s. So in miles one light year is equal to 5.8 x 10^12 miles. And one light year from earth means if light travels from Earth then it will reach to particular destination after one year.

Moreover astronomical unit (AU) is also commonly used. 1 au is equal to average distance between earth and sun. So approximately distance of Sun from mercury is 0.39 AU.

Hope it helps.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.